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CROSS Safety Report

Post-tensioned precast concrete tank failure

Report ID: 147 Published: 1 July 2009 Region: CROSS-UK

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Overview

A utility company suffered a sudden and catastrophic failure of a concrete tank at a sewage treatment works.

Key Learning Outcomes

For all built environment professionals:

  • The utility company who own the facility recommend erring on the side of caution and do not recommend the use of such tanks unless a system of protecting the cables from corrosion is in place and the possible influence of the grease is clarified

Full Report

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A utility company suffered a sudden and catastrophic failure of a concrete tank at a sewage treatment works. The tank was constructed from precast concrete panels which were prestressed with circumferential un-bonded cables in grease filled sheaths. They have a large number of similar tanks.

A photograph taken after the collapse shows the concrete panels lying around the base in a star formation (Figure 1). All parties who have investigated this incident agree that the ultimate failure was caused by stress corrosion cracking, but it is not possible to say exactly what caused it. At some stage there was pressure water testing of the cable ducts which may have had an effect on subsequent performance.

Image
Figure 1: aerial view of failed precast concrete tank

The design appears to rely on the grease and the sheathing to provide its corrosion protection, and it would not be expected that grease used for this purpose would emulsify in water. Tanks of this nature, which are built either above ground or partly buried, rely entirely on the tendons for their structural integrity.

The utility company who own the facility therefore recommend erring on the side of caution and do not recommend the use of such tanks unless a system of protecting the cables from corrosion is in place and the possible influence of the grease is clarified.

Expert Panel Comments

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Failures with unresolved causes are difficult to give advice on and this warning by the utility company should be taken seriously by owners, designers, and suppliers of such tanks. Of particular concern are failures which may be generic but not display signs of distress before collapse. CROSS would be very interested in hearing of similar concerns.

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